In a cellular wireless system, a service area is divided into a number of coverage zones generally referred to as cells. Each cell may be further subdivided into a number of sectors. Wireless terminals in a cell communicate with the base station that serves the cell. Wireless terminals may include a wide range of mobile devices including, e.g., cell phones and other mobile transmitters such as personal data assistants with wireless modems.
A problem with known cellular communications systems is that transmission by wireless devices in one sector of a cell may collide with transmissions by wireless devices in an adjacent sector of the same cell or in a neighboring cell. For example, orthogonal frequency division multiplexed (OFDM) systems take a given bandwidth and splits it into a number of evenly spaced tones that can be used to transmit data. When transmissions by devices in overlapping sectors and/or cells use the same tone or set of tones, multiple collisions may occur over a period of time due to the operation of devices in adjacent sectors and/or neighboring cells. This problem is particularly noticeable where transmissions are periodic or nearly periodic.
In periodic or nearly periodic situations, mutual interference caused by wireless terminals in adjacent sectors and/or cells may be highly correlated. For example, when a tone assigned to a wireless terminal A corresponding to a first sector is the same as a tone of another wireless terminal B corresponding to an adjacent sector, in the next transmission period, the tone of wireless terminal A will again be the same as wireless terminal B in the case where the tones are assigned using the same function and recur periodically. Correlated interference of this type can cause signals transmitted by the same two wireless terminals to repeatedly interfere with each other over a long period of time. If the two interfering wireless terminals are disadvantageously located, e.g. in a boundary region between two adjacent sectors, the base station's receivers for each sector may not be able to detect the signals correctly from the two interfering wireless terminals for a long period of time.
In order to reduce the risk of correlated or prolonged interference it would be beneficial if it was possible to assign tones to devices in neighboring sectors and cells in a manner that would minimize the risk of correlated interference.
Some approaches use a cell identifier in controlling the hopping sequence used by mobiles. The mobile may detect the cell identifier and then use it in a hopping equation to control uplink hopping. By using different cell identifiers at neighboring base stations, hopping can be controlled using a hopping equation based on the cell identifier.
As the demand for cell capacity has increased, sectorizaton of cells has grown in importance. This has complicated the problem of how to implement and control hopping in cells to achieve a desirable result, e.g., control the collisions between cells. With the advent of sectors, there has developed a need to control hopping not only with respect to other cells but with respect to neighboring sectors within a cell. While assigning sectors individual identifiers which can be used as a single control value in controlling hopping sequences as was done previously with cell identifiers, given the relatively large number of sectors in a system, the length of the identifier would need to be larger than in the case of where a cell identifier was previously used, if the same number of cells is to be supported in a system.
For a variety of reasons, using large sector identifiers can be undesirable. Furthermore, for a variety of reasons, it can be desirable to have separate cell and sector identifiers with the cell and sector information. This allows for the cell and/or sector information to be transmitted differently and/or at different rates of frequency.
In view of the above discussion, it should be appreciated that it would be desirable if method and apparatus could be developed which would allow wireless terminals to control tone hopping based on a combination of a cell identifier and a sector identifier while still achieving both highly predictable and desirable collusion properties with respect to transmissions by adjoining sectors of the same or neighboring base stations.
In view of the above discussion, it becomes apparent that there is a need for minimizing the potential for collisions between transmissions that occur in adjacent cells and neighboring cells of a wireless communications system which supports the use of multi-sector cells. It is desirable that the probability that transmissions from any given device in adjacent sectors or neighboring cells will collide repeatedly be minimized to avoid extended periods where communication signals are blocked for any particular device.